The present invention relates to a process for charging latent heat storage devices, and more especially to a process for charging latent heat storage devices with salt hydrates.
In the utilization of thermal energy, storage thereof (i.e., the station between the supply or production of energy and the demand or consumption of it) is of great importance. This is true to an even higher degree in the utilization of low temperature heat, such as may be obtained, for example, from solar energy or from the heat of the environment. It is obvious that particularly in these cases there is a disparity between the supply and the demand of the energy.
The advantages resulting from the utilization of latent heat, for example, the heat of melting, in contrast to storage in conventional water or packed reservoirs, have been discussed in recent years on a relatively broad basis, in particular for applications in low temperature thermal systems, and have lead to the concept of latent heat storage devices. Principally, salt hydrates with suitable melting points have been proposed and investigated; see for example VDI-Berichte, 288, 79 (1977).
Various systems are conceivable as storage devices and are mentioned as such in the literature. On the one hand, they may consist of containers bounded by heat exchangers and designed as flat as possible in order to prevent interference with the transport of heat. To eliminate stratification effects which could lead to the precipitation of lower hydrates, the lamellar subdivision of the container or packaging into flat bags (as disclosed in DE-OS No. 22 23 882) has been proposed. Jacketing with synthetic plastic materials according to DE-OS No. 27 41 829 is also known. Furthermore, packing of the storage medium into synthetic plastic spheres or similar containers, such as those used for the cooling of beverages, is conceivable. In such cases, it is possible lto design volume into the storage tank in the third dimension also, as here the transport of heat is effected by other liquids (e.g., oil, water). Furthermore, constant intermixing of the storage medium in the liquid state, by means of a heat carrier liquid which bubbles through the melt but is immiscible with it, has been described in "Tagung der Deutschen Gesellschaft fuer Sonnenenergie", Proceedings volume III, 1977, page 80.
In DE-OS No. 26 58 120 and No. 27 20 188, containers with flexible walls or with expanding heat transport liquids are proposed to equalize the volume changes occurring during phase changes.
Heretofore, molten salt hydrates were used for charging of the storage devices. This process is less than favorable for a number of reasons:
During the melting of crystalline solids in the powder form, very high resistance to the transport of heat, among others by the entrapped air, must be overcome. These conditions are aggravated by sintering or partial melting processes associated with volume changes.
Heat transfer is extremely slow also during the melting of more compact salt hydrate in large containers.
Strongly hydrate-containing salts, for examples, Glauber salt or disodium-phosphate-dodecahydrate, have a very strong tendency to decompose under certain conditions in storage. This leads, on the one hand, to losses of water which are difficult to determine and therefore equally difficult to compensate for, and on the other hand, frequently to such a stubborn sintering of the salt that mechanical comminution of the material prior to the filling of the storage device or the melting vessel becomes unavoidable.
Salts containing water of hydration, for example, disodiumphosphate-dodecahydrate, are frequently more difficult to prepare, because of unfavorable thermodynamical data and relatively low melting points, than lower hydrates, such as, for example, disodiumphosphate-dihydrate. Naturally, this is reflected by the cost of production.
The highly different volumes of commercially available salts and their resulting melts render multiple refilling of the melting or storage vessels or a very large scale design of the melting vessel necessary.
In the course of the melting process, relatively large temperature differences are developed between the nonhomogeneous areas, which readily lead to stratification effects and consequently to the precipitation of lower hydrates. This makes it necessary to homogenize the melt by means of intensive agitation after melting. The end of the melting process can, however, be detected only very difficultly.
It is not economical to melt a salt hydrate with a high input of energy. The dissolution of lower hydrates or of anhydrous salts (U.S. Pat. No. 2,677,367) in water also requires the introduction of heat in order to reach the phase transformation temperature.